This application relates to power steering systems, and more particularly, to magnetic actuators for use with variable effort power steering systems.
For example, power steering systems in motor vehicles, etc. are designed to provide appropriate hydraulic or electrical assist to allow a driver to complete a turn of the motor vehicle. The driver applies a steering input through a steering wheel that is rotationally connected to a first shaft. The first shaft is rotationally coupled to a second shaft that is, in turn, connected to a steering mechanism. The first and second shafts are torque transmittingly coupled together by a compliant member, such as a torsion bar. The torsion bar allows the first shaft to rotate with respect to the second shaft by a predetermined number of degrees, e.g., +/−12 degrees. Mechanical stops prevent further movement. The amount of steering assist applied to the steering mechanism is determined as a function of the degree of torsional strain or movement in the torsion bar.
An exemplary power steering system is a hydraulic variable effort steering device having a proportional control valve and a magnetic actuator for varying the effective compliance of the torsion bar. The proportional control valve includes a valve spool connected to a manual steering wheel, a valve body connected to wheels of the vehicle, and a torsion bar between the valve spool and the valve body. Throttling orifices are positioned between the valve body and the valve spool to regulate a steering assist boost pressure when the valve spool is rotated relative to the valve body from the center position by manual effort at the steering wheel. As a result, a centering torque in the torsion bar is induced to effect a tactile response for the driver in the steering wheel. The magnetic actuator includes permanent magnets arranged around a ring that is attached to the valve spool. It also includes a pole piece attached to the valve body having outer pole teeth outside of the magnetic ring, inner pole teeth inside of the magnetic ring, and an exciting coil magnetically coupled to the pole teeth. The inner teeth and outer teeth are connected by a non-magnetic portion so that the inner teeth and outer teeth are magnetically decoupled. A current can be applied to the exciting coil to induce an electromagnetic torque between the pole piece and the permanent magnetic ring and thus increase or decrease the effective torque of the torsion bar depending on the direction of the current flowing through the coil.
The arrangement and shape of the teeth in current magnetic actuators are often altered to achieve a desired torque. The teeth in current magnetic actuators are arranged in groups. In some cases, the teeth in each group are profiled differently, the inner teeth and the outer teeth are mis-aligned, and/or the arrangement of the groups of teeth around the magnetic ring is asymmetrical. Additionally, the magnetic actuators are often larger in size than desired due to magnet utilization being poor. As a result of these design specifications, current magnetic actuators can be difficult and expensive to manufacture.
It is therefore desirable to develop an improved magnetic actuator that is smaller in size and easier to manufacture.